The use of radar to detect threats to the wingtips of aircraft faces the problem that, for large aircraft with wide wingspans, many features commonly found around airports, such as ground vehicles and personnel, lane markers, and runway drains, may all serve to trigger false threats when the wing, and the sensor, approach or pass over them. Anticollision systems must have very low false-alarm rates to be useful to the operators. The use of very narrow beam patterns to distinguish threats fails due to the general ratio of wing height versus range, which can be as much as 100:1, see FIG. 1. So the ability to actually determine the height of the object, or in the case of hangar opening proscenia (the upper clearance) becomes very important.